1. Field of the Invention
The present invention relates to a semiconductor package, and more particularly, to a power module package having excellent heat transfer characteristics.
2. Description of the Related Art
Generally, a semiconductor package is manufactured in the following way: one or more semiconductor chips, such as power semiconductor devices or integrated circuits, are mounted on a lead frame or a printed circuit board (PCB), then sealed with an epoxy molding compound (EMC) for protecting the chips, and the packaged chips are mounted on a mother board or a PCB for a system. As used hereinafter the word “chip” means a semiconductor power device or a semiconductor integrated circuit. A semiconductor power device may be a single power transistor or one or more power transistors including one or more transistors for controlling or monitoring operation of the power transistors.
While integrated circuits and other electronic apparatus have long experienced demands for high speed, high capacity, and high levels of integration, now power devices such as those applied to automobiles, industrial apparatus, and home appliances are also confronting similar demands for reduced size, lower weight and low cost. One way of resolving these demands is to construct a power module package that contains two or more semiconductor chips in a single semiconductor package. Such power module packages include one or more power circuit chips and a control circuit chip. But power circuit chips generate much more heat than the heat generated by integrated circuits or control chips. Therefore, effectively transferring heat from the chips to outside the package is critical for maintaining high reliability for a long time for such power modules.
The U.S. Pat. No. 5,703,399 by Majumdar, entitled “Semiconductor power module” discloses a power module package having a heat sink and FIG. 1 herein illustrates a cross-sectional view of the power module package shown in that patent. Referring to FIG. 1, the power module package 10 mounts a plurality of semiconductor chips constituting a power device 9 and a control device 8 on a lead frame 3 that has a heat sink 1 below the lead frame 3. The package shown In FIG. 1 has two types of EMC. Reference numeral “2” represents a “lower EMC” having excellent thermal conductivity and ordinary electrical insulation and upper EMC 7 has ordinary thermal conductivity and excellent electrical insulation. The power circuit chip 4a is mounted on one side of the lead frame 3 and a control integrated circuit chip 5a is mounted on the same side of the lead frame 3 and spaced from the power chip 4a. Reference numerals 5b, 6b and 6a represent, respectively, a resistance component, a gold wire, and an aluminum wire. In the power module package 10 having the construction as described above, heat generated from the power circuit chip 5a is mostly delivered to the heat sink 1 through lower EMC 2 and then to outside the power module package 10 through the heat sink 1. According to the above United States patent, the heat sink 1 is made of a metal having high thermal conductivity such as copper or aluminum.
When, as in the package 10 where the heat sink 1 is manufactured using electrically conductive material such as metal, the lower EMC 2 should satisfy the following two conditions. First, heat transferred from the power circuit chip 4a should be transferred quickly to the heat sink 1. Second, the lead frame 3 should be electrically insulated from the heat sink 1.
To satisfy these conditions, the above United States patent uses an EMC with high thermal conductivity for the lower EMC 2. However, even though the EMC has high thermal conductivity, its thermal conductivity is 2 W/m·K, which is much less than the thermal conductivity of an aluminum heat sink 1 whose thermal conductivity is 100 W/m·K. In order to provide sufficient electrical insulation between the heat sink 1 and the lead frame 3, the lower EMC 2 should be at least 500 μm thick or more so that the lead frame may be insulated from the heat sink 1. If the EMC 2 is much thinner, then one or both devices 4a, 5a may short circuit to the heat sink 1 and damage or destroy the power module 10. As such, the heat sinking ability of the power module package 10 is limited by the lower EMC 2.
The power module package 10 uses two EMCs 2 and 7 and each has different properties. Those skilled in the art understand that there is often a tradeoff between the electrical insulating ability of a molding compound and its thermal conductivity. In general, as one increases, the other decreases. So, in the package 10 that requires an EMC having high electrical insulation for the upper EMC, a two-stage molding process is performed. Accordingly, a manufacturing process of the power module package 10 is complex and costly. One way of solving the above problem and using only one EMC is shown in Korean Patent Publication No. 2002-0095053, filed by the same applicant as the present invention, and entitled “Power module package having improved heat emission capability and method thereof.” FIG. 2 herein illustrates a schematic, cross-sectional view of an example of the power module package 100 suggested by the above Korean Patent Publication.
Referring to FIG. 2, the power module package 100 mounts a power circuit element 120 and a control circuit element 130, both on a first surface 111 of the lead frame 110. The power circuit element 120 is mounted on a down-set (recessed) die pad 140 of the lead frame 110. A heat sink 150 is attached to a second surface 112 of the down-set die pad 140 by a high temperature tape 160. In FIG. 2, reference numerals 121, 122, 130, 132, and 170 represent one or more power circuit chips, an aluminum wire, a control circuit chip, a gold wire, and an EMC, respectively.
In the power module package 100, a heat sink 150 made of ceramic is directly attached to a backside of a down-set die pad 140 by a high temperature tape 160. The high temperature tape 160 can be as thin as about 50 μm. Since the sealing process for the power module package 100 is performed using only one EMC 170, the manufacturing process for the package shown in FIG. 2 is simpler than the process for the package of FIG. 1 and the process to make the package of FIG. 2 can be automated to further reduce cost.
However, the ceramic heat sink 150 has a thermal conductivity of about 24 W/m·K, so that its heat sinking ability is not as good as metal, and further, ceramic is more expensive than metal. Still further, there is limit on how thin one can make a ceramic heat sink because ceramic is brittle and will crack if it is too thin.
FIG. 3 illustrates a cross-sectional view of another example of a power module package 200 disclosed in the above-described Korean Patent Publication No. 2002-0095053. Referring to FIG. 3, the power module package 200 uses a direct bonded copper (DBC) substrate 250. The DBC substrate 250 includes: a ceramic plate 251 at the center; an upper copper layer 252 attached to an upper surface of the ceramic plate 251; and a lower copper layer 253 attached to a lower surface of the ceramic plate 251. One or more power circuit chips 221 are mounted on the upper copper layer 252 and the lower copper layer 253 acts as a heat sink of the power module package 200. Reference numerals 210, 222, and 270 represent a lead frame, an aluminum wire, and EMC, respectively.
According to the power module package 200, the upper and the lower copper layers 252 and 253 are directly attached to the ceramic plate 251 without using EMC (refer to the reference numeral 2 in FIG. 1) or a high temperature adhesive (refer to the reference numeral 160 in FIG. 2) and the heat dissipation capability of the heat sink 250 is excellent thanks to high thermal conductivity of copper. Further, since the copper layers 252 and 253 are attached to the upper and lower surfaces of the ceramic plate, problems caused by brittleness of the ceramic are overcome. Still further, since the encapsulation process for the power module package 200 is performed in a single transfer molding process using one EMC 270, its manufacturing process can be simplified and automated to reduce costs.
However, the ceramic plate 251 of DBC substrate 250 still has a lower thermal conductivity than metal and the ceramic plate 251 is still about 635 μm thick so that the manufacturing cost of the DBC process is high. As such, there is still substantial room for reducing the size and improving thermal dissipation ability of the power module package 200.